Acrylic resins, having excellent optical characteristics, are widely employed as a material for lenses, illumination devices, automobile parts, displays, optical-semiconductors, etc.
When acrylic resin is formed into a plate or the like, a composition containing (meth)acrylate monomer(s) is cast into a mold and polymerized therein. This method is known as cast polymerization. During cast polymerization, in order to prevent leakage of the raw material composition through the mold, enhance curing rate, and reduce shrinkage in curing, the viscosity of the composition is generally adjusted to a desired value. In this case, a viscous liquid, which is called a syrup, is often used to control the viscosity of the composition.
A syrup for producing acrylic resin is prepared through a known method which includes dissolving a separately produced polymer (e.g., poly(methyl methacrylate)) in a raw material monomer or through a known method which includes partially polymerizing a raw material monomer and dissolving the polymerized product in the raw material monomer (Patent Document 1).
By virtue of its excellent transparency, acrylic resin is often used in the vicinity of a light source. In this case, if the acrylic resin has poor heat resistance, yellowing, reduction in light transmittance, and deformation may occur. In order to prevent these phenomena, there has been proposed a composition containing a (meth)acrylate monomer having a substituent of a tricyclodecane structure, which monomer is thought to have a comparatively high glass transition temperature, and a polyfunctional acrylate of the (meth)acrylate (Patent Document 2).
A syrup produced from a (meth)acrylate monomer having such an alicyclic hydrocarbon substituent has drawbacks. Specifically, since poly(methyl methacrylate) and a polymer of a methacrylate having a tricyclodecane structure are not readily dissolved in a (meth)acrylate monomer having alicyclic hydrocarbon substituent, the mixture must be heated, or kneaded by means of a kneader for dissolution. In addition, since such a methacrylic polymer is difficult to dissolve in a (meth)acrylate monomer having alicyclic hydrocarbon substituent, a cured product has poor characteristics (e.g., poor hue). When such a cured product is used under heat or UV light, the hue of the product is further deteriorated.
Meanwhile, a copolymer produced mainly from a methacrylate having an adamantyl group is known to be employed as an optical disk substrate (Patent Document 3). However, there has never been known a syrup for use in production of such a copolymer.    Patent Document 1: Japanese Patent Publication (kokoku) No. Hei 4-75241    Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. 2005-187643    Patent Document 3: Japanese Patent Application Laid-Open (kokai) No. Hei 2-179940